1. Field of the Invention
The present invention relates to a heat detector for use in a fire alarm system and the like.
2. Description of the Related Art
In a conventional heat detector, a thermistor is used as a temperature detection element for detecting ambient temperature around the heat detector. Several methods have been proposed to detect when the thermistor is operating abnormally or not. For example, when the thermistor has an open or is shorted, the output voltage of a temperature detection circuit changes substantially in comparison with the normal output voltage. When this detected output voltage is larger than a predetermined maximum value or smaller than a predetermined minimum value, it is determined that the thermistor has an open or has been shorted.
However, when the characteristics of the thermistor deteriorate due to change by aging, its resistance as measured at a constant temperature will change gradually, and thus the output voltage of the temperature detection circuit will not change abruptly. Such a gradual change in resistance of the thermistor due to the deterioration of its characteristics can not be detected by the conventional techniques.
When the temperature detection range of the heat detector has been set for example to a range higher than -10.degree. C., it will be possible to assume that the deterioration of the thermistor has occurred if a low temperature such as -20.degree. C. is detected. In this case, however, when the actual ambient temperature around the heat detector is for example +20.degree. C., even if an incorrect value such as +10.degree. C. or +30.degree. C. is measured as a result of the deterioration of the thermistor, it is impossible to determine that the measuring result is incorrect. That is, the conventional technique suffers a drawback in that the deterioration of the thermistor can not be detected for the entire operation temperature range.
When an element such as a transistor or a diode other than a thermistor is used as the temperature detection element, a problem also arises in that the deterioration of the temperature detection element can not be detected for the entire operation temperature range.